The proposed research is designed to use microelectrode and micropuncture techniques in the rat kidney to evaluate 2 current hypotheses regarding the fate on the CO2 produced in the proximal tubular lumen in the process of bicarbonate reabsorption. In the first hypothesis, the CO2 produced in the lumen is consumed in the cell in the generation of new bicarbonate; in the second hypothesis, the CO2 crosses the cell and directly increases peritubular capillary PCO2. We plan to evaluate these hypothesis in terms of two CO2 pressure gradients that have been detected in the rat kidney: 1) a small transepithelial gradient, and 2) a large capillary-arterial gradient. In the proposed experiments, we will measure tubular and capillary PCO2, proximal tubular bicarbonate reabsorption, renal blood flow, and renal oxygen consumption. The latter two parameters are measured because metabolic CO2 production and blood flow can potentially influence PCO2. Computer models of the predictions of the 2 hypotheses have been developed to help in the design of experiments to differentiate between the hypotheses. Free-flow micropuncture studies will be carried out to evaluate these PCO2 gradients under conditions in which bicarbonate reabsorption, arterial PCO2, renal blood flow and hematocrit are altered. In situ microperfusion of individual nephron segments and peritubular capillaries will also be carried out to evaluate the PCO2 gradients. Interaction between our experimental results and the computer models should allow us to differentiate between these two hypotheses, and to develop a more complete model of the process by which bicarbonate reabsorption is accomplished.